Interferometric signal analysis involves the determination of signal wavelength, wave velocities, distances and directions using interference phenomena between two signals Particular application may include optical signal analysis in a fiber optic gyroscope (FOG).
A FOG includes a light source, e.g., a laser diode, which provides coherent or semi-coherent light split into two substantially equal waves which counter-propagate in a coil of fiber optic waveguide. The waves are recombined interferometrically at the coil output such that the light intensity seen by a detector depends on the relative phases thereof.
When the coil rotates about a normal axis, the waves take different times to traverse the coil. This non-reciprocal phenomenon, known as the Sagnac effect, causes a change (shift) in the relative phase between the waves at a detector and, therefore, a change in the light intensity signal at the detector. Depending on the initial phase difference, which can be controlled by, e.g., application of suitable phase modulation at one end of the coil, the magnitude and direction, e.g., increase or decrease, of the change in the light intensity signal are dependent upon, respectively, the rate and sense of the rotation applied to the coil about the axis.
The detector signal intensity is a cosine function, and, thus, is relatively insensitive to small rotation rates. It is known to induce an optical phase modulation (phase dither) at a relatively high frequency, e.g., a sinusoid or square wave, on the counter-propagating waves to both increase FOG sensitivity and to determine the change in direction of FOG rotation. Additionally, control of the modulation frequency at the relatively high frequency (typically at the loop eigenfrequency) shifts the measurement bandwidth away from lower frequencies subject to excess noise. A phase modulator at one end of the coil is driven at a frequency corresponding to the coil eigenfrequency. The detector signal intensity is a function of both the modulation and rotation induced phase differences between the counter-propagating waves.
However, the eigenfrequency is dependent upon physical characteristics of the coil, including length and index of refraction of the coil optical waveguide. These characteristics may vary with changes in environment, e.g., temperature, thereby causing changes in the eigenfrequency. Variations in the modulation frequency from the coil eigenfrequency cause errors in the measured rotation rate signal which may render the FOG unsuitable for use in high accuracy applications.